The natural gas used by consumers is composed almost entirely of methane. However, natural gas found at the wellhead, although still composed primarily of methane, is by no means as pure. Natural gas is typically isolated from three different sources: oil wells, gas wells and condensate well, but whatever the source of the natural gas, it commonly exists in mixtures with other hydrocarbons; principally ethane, propane, butane, and pentanes. Each of these hydrocarbons has a similar size and polarity. Pentane, a larger hydrocarbon, is more easily separated, but for the smaller hydrocarbons, separation is a challenging endeavor.
Most membranes used in the gas separation field are derived from glassy polymers and are generally unacceptable for heavy hydrocarbon separations. Most glassy polymers have high permeation of methane relative to propane, butane, and other gases, and these glassy polymers are not sufficiently selective to discriminate between heavy hydrocarbons and methane. Glassy polymer membranes have been used to effectively separate oxygen and nitrogen in air samples, and have also been used to separate butanol from other biofuels; however, these glassy polymer membranes achieve low hydrocarbon selectivity and thus do not provide sufficient separation for natural gas upgrading applications. In contrast, rubbery polymers, such as polydimethylsiloxane (PDMS), have high permeability, but generally have a lower selectivity. Moreover, the glassy polymer membranes tend to show a decrease in performance due to aging. Aging occurs from collapse of free volume, which tends to cause lower permeability.